506 THE AGE OF MAMMALS 



the large herds of the domesticated horses and cattle on the western plains. 

 Under these conditions horses are driven to food, such as the branches of 

 willows, which is very deleterious to them. Under the influence of hunger 

 cattle will also feed eagerly and indiscriminately on plants which mpy be 

 injurious to them or to their young, as recorded by Chestnut and others in 

 the United States Agricultural Department. After heavy snowstorms, when 

 the grass is covered with snow, it often happens that only the taller species of 

 plants are exposed.^ In such cases the poisonous larkspurs {Delphinium 

 glaucum) are greedily eaten by cattle which would otherwise avoid these 

 plants. This tendency is increased by the fact that ruminants do not feel 

 at ease so long as the stomach is not full, and are inclined to eat anything 

 in sight after a snowfall. 



Similarly, enforced migrations among wild as among domesticated 

 animals might cause them to become less fastidious about their food. It 

 is observed ^ among domesticated animals that when feeding quietly on the 

 range they exercise considerable choice in the selection of forage plants, 

 but wheji driven six or eight miles daily they are frequently forced by 

 hunger to bite off almost all kinds of plants which grow along their course. 

 Animals vary greatly in adaptability to new conditions caused by long 

 cold and heavy snowfall; horses remove snow even to a depth of three or 

 four feet, and find food sufficient to carry them through the winter, while 

 cattle under the same conditions starve. 



Forestation, deforestation, and reforestation. — It is certain that the Hol- 

 arctic region, or circumpolar belt, was forested even to the shores of the Arctic 

 Ocean in early Pleistocene times. The remains of large extinct quadrupeds 

 in this region are almost everywhere associated with evidences of forests, 

 and of forest-frequenting animals, such as the beaver. The forests naturally 

 furnished the necessary conditions of life of certain quadrupeds, especially 

 of browsing animals, and even of Proboscidea. Among Artiodactyla the 

 deer, among Perissodactyla the tapirs, are typical forest animals. Condi- 

 tions, therefore, which would cause deforestation would also become the 

 means of diminution, and finally of extinction. Such conditions are intense 

 cold, (a) heavy snow-capping of the tundra regions of the north, (6) the 

 dry cold and dust storms of the steppes. In Europe a period of deforesta- 

 tion and a long unforested period of dry cold certainly succeeded each other. 

 In North America we have evidence of similar conditions in our own loess 

 period, and there certainly occurred a great deforestation in the regions now 

 known as the ' barren grounds,' which pass into the tundras with frozen 

 subsoil to the north. 



After considering the diminution of life in Alaska, Maddren^ summarizes 



' Chestnut, V. K., The Stock Poisoning Plants of Montana U.S. Dept. Agric, Div. 

 Botany., Bull. 26, Washington, 1901. 



^ Maddren, A. G., Smithsonian Exploration in Alaska in 1904, in Search of Mammoth and 

 other Fossil Remains. Smiths. Misc. Coll., Vol. XLIX, 1905, p. 65. 



